/****************************************************************************
 *
 * Copyright 2016 Samsung Electronics All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
 * either express or implied. See the License for the specific
 * language governing permissions and limitations under the License.
 *
 ****************************************************************************/
/************************************************************************
 * libc/math/lib_expf.c
 *
 * This file is a part of NuttX:
 *
 *   Copyright (C) 2012 Gregory Nutt. All rights reserved.
 *   Ported by: Darcy Gong
 *
 * It derives from the Rhombs OS math library by Nick Johnson which has
 * a compatibile, MIT-style license:
 *
 * Copyright (C) 2009-2011 Nick Johnson <nickbjohnson4224 at gmail.com>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 ************************************************************************/

/************************************************************************
 * Included Files
 ************************************************************************/

#include <sys/types.h>
#include <math.h>

#include "lib_internal.h"

/************************************************************************
 * Private Data
 ************************************************************************/

static float _flt_inv_fact[] = {
	1.0 / 1.0,					/* 1/0! */
	1.0 / 1.0,					/* 1/1! */
	1.0 / 2.0,					/* 1/2! */
	1.0 / 6.0,					/* 1/3! */
	1.0 / 24.0,					/* 1/4! */
	1.0 / 120.0,				/* 1/5! */
	1.0 / 720.0,				/* 1/6! */
	1.0 / 5040.0,				/* 1/7! */
	1.0 / 40320.0,				/* 1/8! */
	1.0 / 362880.0,				/* 1/9! */
	1.0 / 3628800.0,			/* 1/10! */
};

/************************************************************************
 * Public Functions
 ************************************************************************/

float expf(float x)
{
	size_t int_part;
	bool invert;
	float value;
	float x0;
	size_t i;

	if (x == 0) {
		return 1;
	} else if (x < 0) {
		invert = true;
		x = -x;
	} else {
		invert = false;
	}

	/* Extract integer component */

	int_part = (size_t)x;

	/* set x to fractional component */

	x -= (float)int_part;

	/* Perform Taylor series approximation with eleven terms */

	value = 0.0;
	x0 = 1.0;
	for (i = 0; i < 10; i++) {
		value += x0 * _flt_inv_fact[i];
		x0 *= x;
	}

	/* Multiply by exp of the integer component */

	value *= lib_expi(int_part);

	if (invert) {
		return (1.0 / value);
	} else {
		return value;
	}
}
